Rotors with permanent magnets are formed with a cylindrical core, usually in iron, which may be laminated or massive and around which is seated a plurality of magnets, usually in the form of arcuate plates, and a rotor cover provided externally to said magnets, in order to retain them close to said core, avoiding relative radial and circumferential displacements between said core and said magnets during the operation of the motor, when the magnets are submitted to centrifugal forces, which tend to separate them from the core, to a momentum, which causes the circumferential displacement of said magnets around the rotor core, and to stresses of thermal origin. Moreover, the constructive form which is used to retain the magnets to the rotor is very important to determine the electrical efficiency of the motor.
Besides having the function of maintaining the magnets close to the core, the cover also avoids the disaggregation and release of magnet fragments caused by the forces existing thereon, as mentioned above. Among the known solutions of a rotor cover for an electric motor discussed in copending Patent Application PI9601676, the wound cover which surrounds the magnets provided around the core and which is obtained by winding a bundle of fibers has, as a constructive advantage in relation to the other known covers, its capability of conforming to the dimensional variations of the magnets, resulting from the manufacturing process of said magnets.
According to the known techniques, the wound covers for an electric motor rotor are obtained by winding a strip or a bundle of fibrous material, which is usually impregnated with resin around the rotor core-magnet assembly and which is cured afterwards during a time and at a temperature which are sufficient to obtain the rigidity required for the rotor cover.
However, during a mass production winding process, which is usually carried out in a sequence of cores carrying respective magnets, the covers thus obtained have the inconvenience of not allowing an appropriate finishing of their end portions. This problem occurs during the passage of the winding element from one rotor to the other in the production line, after the winding of the previous rotor has been concluded. At this moment, the winding equipment causes a sudden movement of the pre-tensioned fiber which forms the cover, towards the next rotor, requiring in the known methods posterior scraping, sanding or machining of each end surface of the wound cover in the rotor, after said cover has been cut, allowing its finishing to take place. Without this finishing, the barbs resulting from the cutting operation cause assembly and durability problems and even impairment in the gap between the rotor and stator.
Besides the above cited problem, during the passage of the pre-tensioned fiber from one rotor to the other, plastic deformation of the mold end element placed adjacent to each of the ends of each rotor may occur, impairing the dimensional characteristics of said rotor and the functional characteristics of said mold end element. One way of eliminating this deformation is to increase the thickness of the mold end element, in order to obtain higher mechanical rigidity.
However, this solution increases the cost of both the material and the process, as well as the weight of the motor.